1. Field of the Invention
The present invention relates to an apparatus and method useful for obtaining improved full arch dental castings according to the lost wax method. The apparatus and method of this invention are particularly useful in that they permit full arch dental castings of extreme accuracy to be prepared using base metal alloys rather than precious metals such as, for example, gold.
2. Description of the Prior Art
Various techniques and forms of apparatus are old and well known for preparing castings according to the lost wax method. Use of the lost wax method within the dental profession for the purpose of casting crowns and other such dental restorations is similarly well known. Of particular note with regard to dental castings is the fact that, dental castings require extreme accuracy in the final product. Accordingly, metallurgical characteristics have virtually dictated that the castings be prepared from precious metals such as, for example, gold. Numerous attempts have been made at preparing dental castings from base metal alloys, but, again primarily due to metallurgical characteristics, base metal castings generally require excessive finish work so that the restoration will fit properly.
Yet another problem inherently present in all casting techniques utilizing the lost wax method is the fact that curing of the investment material is an exothermic reaction. That is to say, as the investment material hardens, heat is generated. This necessarily results in "swelling" of the investment material, and this usually results in at least some compression of the wax model within the investment. The compression against the wax model for a dental restoration will have a undesirable effect, for it will tend to distort the restoration pattern. Then, when the pattern is burned out and the molten metal is cast into the void formed within the hardened investment material, the casting thereby obtained will not correspond exactly to the original pattern. This results not only in discomfort to the patient for whom the restoration has been prepared, but unnecessary expenditure of time, energy and money for the patient, the dentist, and the dental technician.
Insofar as the problem of obtaining accurate castings utilizing base metal alloys is concerned, the only effective solution available today involves either detailed and expensive hand-finishing of the casting, the taking of new impressions and making new castings, or simply accepting a relatively inferior final product. Of course, another solution to this problem would be to prepare all castings from gold rather than a base metal alloy, but this obviously has an adverse economic impact on the patient.
Insofar as the expansion problems associated with the exothermic reaction whereby the investment material solidifies, prior patent literature does offer some suggestive solutions. U.S. Pat. No. 2,337,036 to Erdle discloses the use of a resilient mold for making porcelain dental restorations. While analogous to the field of preparing dental restorations, the resilient mold of this patent would not appear to be suitable for metal castings according to the lost wax process. Summy in U.S. Pat. No. 2,243,445 discloses an expansible absorbent receptacle for investments. U.S. Pat. No. 2,450,567 to Schwartz discloses a two-part flask for retaining investment material. Yet another casting ring constructed and arranged to prevent breakage of an investment made therein is disclosed in U.S. Pat. No. 1,976,655 to Carpenter. While no doubt useful for their stated purposes and in light of their various disclosures, none of these patents truly solve the problems associated with dental castings prepared utilizing the lost wax method and base metal alloys.
While the teachings of these patents do disclose apparatus including investment ring constructions whereby the investment material may expand, the structures disclosed are unnecessarily complicated. Furthermore, none of these prior art teachings make any attempt to solve the problem created by the fact that as the exothermic reaction takes place for solidifying the investment material, and as the wax pattern is subsequently burned out from within the investment material, different areas within the volume defined by the investment experience different temperatures. For the purpose of obtaining consistent, reproducible quality of final castings, it is desirable that the wax pattern, and the resulting casting mold, be formed at about the center of the investment material. While this is relatively easy to accomplish when, for example, a single crown is being cast, problems are encountered if one wishes to cast more than a single crown at a time, or if one is casting a larger restoration such as, for example, a full arch. This problem of variable temperature zones within the investment simply does not appear to have been dealt with in the prior art.
It is therefore apparent that there is a need in the prior art for improved means for preparing full arch dental castings. The need is especially great with regard to the preparation of dental castings from base metal alloys. To provide mechanical stabilization during waxing, manipulating, investing, casting, cooling and subsequent processing a bar interconnecting the distal tooth portions is used. This cross-arch bar may either be formed during the casting process or be pre-formed and attached prior to casting. This cross-arch bar minimizes distortion at the distal ends of the full arch casting and therefore results in more accurate multi unit dental bridge castings. Of course, because utilization of the lost wax process invariably results in destruction of some casting material, it would be further desirable if at least some parts of the improved apparatus were reusable.